Make a "Y" speaker connection. In other words disconnect the RETURNS (Black) of the LEFT and RIGHT speakers from the amplifier, and connect them both to the HI (Red)input of the center speaker. (You can make this wiring at the back of the amplifier, or at the speakers themselves). Then run the RETURN of the center speaker to the power amp RETURN (Black). By the way, this is a "derived" center channel...not "Phantom"

WARNING>>>THIS ASSUMES THAT YOUR POWER AMP HAS BOTH RETURNS AT GROUND. THIS IS OFTEN NOT TRUE FOR DIGITAL AMPS